1. Field of the Invention
The present invention is generally directed to a method and apparatus for reducing the memory required by position measuring systems, and more particularly, an arrangement for reducing the amount of memory required to store interpolation values for position encoders thus making the measurement system adaptable to VLSI technology.
2. Description of the Prior Art
Incremental measuring systems are generally well known in the art. The linear displacement or angular rotation of an element is determined by detecting pulses. Typically, these pulses are summed up in an electronic up-down counter and the measurement value is numerically displayed.
FIG. 1 illustrates an incremental rotational position measuring system. The measurement embodiment comprises a graduated plate 2 with a radial grid 3 thereon connected to a shaft 5. There is also a reference mark 4 on the graduated plate 2. In order to detect the position of the shaft 5, the radial grid 3 is scanned by a scanning unit, the essential components of which are a light source 6, a condenser lens 7, a scanning plate 8 and at least two photodetectors 9.
In operation, as the graduated plate 2 turns, the gaps in the scanning plate 8 and the radial grid 3 periodically align so that a beam of light from the light source 6 impinges upon the photodetectors 9 in accordance with the transmission pattern on the grid 3. The photodetectors 9 thus receive a periodically changing amount of light which is transformed into electrical signals. The two photodetectors 9 are offset from one another by one fourth the graduation period, T/4. This offset creates signals U1 and U2 which are 90.degree. out of phase with each other.
In an evaluating circuit EXE, signals U1 and U2 are digitalized by an A/D converter and interpolated. The interpolation values are determined in advance and are stored in memory which will be further discussed with respect to the present invention. Interpolation increases measurement precision. The output of the evaluating circuit EXE comprises the digitalized and interpolated signals in the form of countable square pulses.
In the simple device illustrated in FIG. 1, the two analog signals have a combination of four (4) zero crossings during one graduation period T. The resolution of the position measuring system is therefore only one-fourth (1/4) of the graduation period. Obviously, to obtain precise measurements, a higher resolution is required and thus the graduation period must be further subdivided.
In Swiss patent CH-PS 4 07 569 a method of increasing the resolution of an interpolating position measuring system is disclosed. The amplitudes of the two signals U1 and U2 are chosen such that EQU U1=A1 sin(2.pi.X/T) EQU U2=A2 cos(2.pi.X/T)
where,
X is the path or angle PA1 T is the signal period so that A1=cos .rho. and PA1 A2=sin .rho.. The two signals U1 and U2 are then added yielding U=sin(.rho.+2.pi.X/T).
Signal U is thus phase displaced to signal U1 by .rho.. Signals can be generated which are phase displaced up to 25 times thus achieving a resolution of up to 1/100 of the scale graduation period.
In order, however, to provide an interpolating position measuring device with such a high resolution, the circuitry becomes more complicated and expensive since for each phase displaced signal, a switching component is needed. This in turn requires that the number of comparators equal twice the subdivision factor.
In European Patent publication EP-A2-0,213,904 an improvement is provided by depositing the interpolation values in the form of an arctangent table in memory. The memory addresses are driven by a code which is formed by the two scanning signals phase displaced to one another. In addition, the publication also discloses that the memory requirement can be reduced to one quadrant of a cycle.
U.S. Pat. No. 4,697,125 discloses that if the analog signals are out-of-phase and sinusoidal, then the interpolation values must be arranged according to an arctangent function to ensure that the output pulses will properly reflect the rotational motion of a point on the plate.
Accordingly, it is a primary object of the present invention to provide a position measuring system that reduces the memory required to store interpolation values thus allowing the system to be implemented with very large scale integration (VLSI) technology.
Another object of the present invention is to provide reduced memory position measuring system that provides signal amplitude information and thus indicates erroneous signals.
Further objects and advantages will become apparent from the following description and the accompanying drawings.